The present invention relates to personnel monitoring systems and to components and methods useful in connection with such systems.
Automated systems have been developed for monitoring persons and verifying the presence of the monitored persons at specified locations. Using such systems, a person can be required to remain in his home or at some other specified location either continuously or during specified hours of the day. Such a requirement may be imposed as a punishment for crime or as a condition of probation, parole or other conditional release from incarceration. A sentence incorporating such a requirement may be employed either as an alternative to incarceration in a conventional jail or as an alternative to an ordinary parole or probation program. Such sentencing avoids the costs and adverse social effects associated with conventional incarceration but still provides effective control of the monitored persons.
Many personnel monitoring systems employ an encoded tag secured to each individual to be monitored. Each such tag may be equipped with a small, battery-powered radio transmitter arranged to broadcast an encoded, radio frequency tag signal. Ordinarily, the tag signal transmitter is switched on only during relatively brief, infrequent intervals such as for a few milliseconds every thirty seconds so as to send the tag signal only in discrete bursts or intervals. This conserves battery power and minimizes interference with other devices.
A receiver or monitoring unit adapted to receive the tag signals may be placed at each monitoring location. When the monitored person leaves the monitoring location, he takes the tag out of radio transmission range so that the monitoring unit no longer receives the tag signal. The monitoring unit thus can detect when the monitored person leaves his assigned location. Depending upon the system design, the monitoring unit can make a record of such departures for later retrieval or else can immediately notify a central monitoring station by sending an alarm signal via telephonic or other communications.
Such a system could be defeated if the monitored person were able to remove the tag from his person and depart from the monitoring location while leaving the tag behind. In that event, the monitoring unit would continue to receive the tag signal and hence, could not detect unauthorized absences of the monitored person. To preclude such cheating, tag signal transmitters typically have been secured to the monitored person by straps passing around the arm or leg of the person to be monitored so that the tag cannot be removed from the person's body without severing the strap. Various schemes have been devised for detecting severance of the strap or otherwise detecting removal of the tag from the person's body and altering the tag signal sent by the transmitter so as to indicate that tampering has occurred. A latch is provided having a normal state and a tamper state, and some sensing arrangement is arranged to trip the latch from its normal state to its tamper state upon tampering. The latch is arranged to remain in its tamper state after such triggering. The tag signal sending means or transmitter is arranged to send a normal tag signal when the latch is in its normal state, and to transmit a different "tamper" tag signal when the latch is in its tamper state. For example, in a multiple bit digital tag signal, one or more of the bits may be "tamper" bits having a first value in the normal signal and a second, different value in the tamper signal.
One scheme which has been utilized heretofore to detect tampering and to trip the tamper latch employs a conductor embedded in the strap which secures the tag transmitter to the monitored person. The conductor forms part of a severance detection circuit, and a small electrical current is continually passed through this circuit. The circuit is responsive to cessation of the current flow to trip the tamper latch into its tamper condition. Thus, if the strap is broken or removed from the tag transmitter, the circuit is interrupted and the latch is tripped to the tamper state. Systems of this sort are disclosed for example in U.S. Pat. Nos. 3,806,874 and 4,885,571.
These systems suffer from a fundamental drawback in that some means must be provided for resetting the tamper latch from its tamper state to its normal state after the tag has been fitted to the person to be monitored. The strap or other securement must be open when the device is initially fitted to the monitored person, so that the tamper latch ordinarily is in the tamper state or in another abnormal state when the device is first fitted to the monitored person. Thus, devices incorporating such a continuous current flow severance detection circuit may include a magnetic reed switch concealed within the housing of the tag and a circuit responsive to actuation of the reed switch to reset the tamper latch. Such systems have been susceptible to cheating by the monitored person. When the device is first fitted and an authorized person resets the tamper latch, the monitored person may observe the officer and deduce that a magnet is used to reset the tamper latch. Armed with that knowledge, the monitored person may be able to reset the tamper latch at will and hence may be able to remove the tag from his person and reset the tamper latch so that the tag continues to emit the normal tag signal.
Moreover, systems of this general design have been susceptible to cheating by short-circuiting the securement system conductor. Typically, the conductor in the strap or other securement device is connected to the remainder of the circuit by concealed terminals. It is difficult to insert a conventional metallic conductor into these terminals so as to "jump" the securement strap conductor. A determined individual may be able to establish a relatively high impedance, but nonetheless effective, electrical connection between these concealed terminals by immersing the entire tag in water or other conductive liquid. With that done, he may be able to sever or remove the securement strap and its conductor without tripping the tamper latch into the tamper state. The high impedance current pathway through the conductive liquid serves as a substitute for the conductor in the securement strap. With the small continuous current flow, the high impedance current path provided by the liquid may appear to be a closed circuit.
One system which avoids these drawbacks is taught in copending, commonly assigned U.S. patent application No. 07/200,088, filed May 27, 1988, and entitled, "Secure Personnel Monitoring System, " now abandoned and refiled as U.S. patent application Ser. No. 07/566,307. As set forth in the '088 application, the tamper detection circuitry of a personnel monitoring tag may incorporate a pair of conductors extending lengthwise along the strap or other securement device but electrically insulated from one another. An electrical potential may be continually applied between these two conductors, but without any current flow therebetween during normal operation. If an attempt is made to sever the strap, the conductors will contact one another, current will flow indicating the severance and tripping the tamper latch. The mechanical configuration of the securement strap and the tag housing may be selected so that the strap cannot be readily detached from the housing without destroying it and without establishing a circuit between the two conductors. In this arrangement, the tamper latch is not set to its tamper condition when the strap or other securement device is initially in an open, unjoined condition before attachment to the monitored person. Accordingly, there is no need to reset the tamper latch after attaching the device to the monitored person. The tag therefore need not incorporate any externally actuable resetting device. These features materially enhance the security of the system. Nonetheless, further improvement, beyond that afforded by the '088 application would be still more desirable.
The problems encountered in design of a personnel monitoring tag are magnified because of the severe cost constraints on such devices. Personnel monitoring tags are utilized in large numbers by governmental authorities, and cost is a significant consideration. Accordingly, there have been substantial, unmet needs for further improvements in personnel monitoring tags and in related devices and methods.